Harmonic Distortion in Cochlear Models

Abstract

The envelope over the train of waves traveling along the cochlear partition is asymmetrical: the distal slope being steeper than the proximal one [G. von Békésy, J. Acoust. Soc. Am. 19, 452 (1947)]. In experiments on models, this asymmetry was seen to increase with intensity. Since the mean revolving velocity of Békésy's “eddies” also varies with intensity, artificial “eddies” were introduced into the model while the intensity remained constant. Thereby the asymmetry of the envelope was also increased. Propulsion of the “perilymphatic” fluids by the “eddies” converts particle motion (ordinarily in closed orbits) [Juergen Tonndorf, J. Acoust. Soc. Am. 5, 558 (1957)] into cycloids. As the eddies accelerate along the partition (see the paper by Tonndorf), the cycloids expand with distance, its velocity attaining a sawtooth-like pattern. The asymmetry of the envelope can be explained satisfactorily by the combined effect upon the partition of the distorted cycloids in both “perilymphatic scalae.” However, the same theoretical considerations lead to asymmetry of the displacement pattern of the partition and to an effect similar to peak clipping, both of which increase with distance. This distorted displacement pattern in the distal region of the partition in turn affects particle motion in the adjacent fluids, whence the distortion is propagated proximally by the eddies to be resolved in the usual way (see the paper by Tonndorf) along the cochlear partition. [This research was supported in whole or in part by the U. S. Air Force, under Contract No. AF-41 (657)148, monitored by the School of Aviation Medicine, UASF, Randolph Air Force Base, Texas.]